Generally, in a switching regulator which converts a DC power-supply voltage into another DC voltage, the restraint of inrush current or excessive current has been one of the significant challenges. Here, the inrush current refers to the current that flows abruptly to charge the capacity for smoothing the output voltage when a switch connected to the power source is turned on in the initial operation such as at power-on. The excessive current, on the other hand, refers to the current that flows excessively from the power source towards the load when the switch connected to the power source is in the ON state. Without restraining the inrush current and the excessive current, the current flowing through the inductor within the smoothing circuit would exceed the maximum rating value, thereby causing the deterioration of the performance or even a breakage. It must be noted that in the present specification the inrush current and the excessive current are collectively called inrush current except for the case where it might be misleading.
In order to restrain the inrush current, the following circuit structure has been suggested.
FIG. 11 shows an example of the circuit structure of a conventional switching regulator. In this example, an inrush current detection circuit 50 includes a resistance element 51 for inrush current detection and an inrush current detector 52 which detects the presence or absence of the inrush current by means of the voltage across the resistance element 51. The inrush current detector 52 places the switching element 53 with a small on resistance in the ON state in a steady operation, and turns the element 53 off to flow the current to a resistance element 54 for inrush current restraint upon the inrush current detection. The resistance element 54 for inrush current restraint is composed of a resistor with a large resistance value or a temperature fuse resistor which interrupts a current flow in response to overheating (Refer to Japanese Laid-Open Patent Application No. 9-121546).
FIG. 12 shows another example of the circuit structure of the conventional switching regulator, which controls the switching pulse width variably with a sawtooth wave in the initial operation. Under the direction of a switch control circuit 57, a selector 56 selects and outputs the sawtooth wave in the initial operation and a reference potential in the steady operation. Consequently, as shown in FIG. 13, early in the initial operation, the on signal of the p-type MOS transistor switch SW1 has a short pulse width due to the comparison between the sawtooth wave and the output voltage Vout, and later the pulse width becomes gradually longer with increasing output voltage Vout. As a result, the inrush current in the initial operation is successfully restrained.
FIG. 14 shows a further another example of the circuit structure of the switching regulator which restrains the inrush current in the initial operation by means of a sawtooth wave in the same manner as in FIG. 12. In the structure shown in FIG. 14, the on-off control of switching elements 61, 62 are done by using a loop 1 in the initial operation and a loop 2 in the steady operation. A sawtooth wave generating circuit 63 generates a sawtooth wave STW so as to perform the initializing operation while gradually changing the switching on-off duty, thereby restraining the inrush current in the initial operation (Refer to Japanese Laid-Open Application No. 5-56636).